For coastal city dwellers, I think it's likely that marine traffic is a major source of the hum, but the world hum map has a lot of reports from far inland that aren't explained by that.

I used to live in a place that overlooked a shipping channel. I could perfectly tell you when a tug boat was near based on the low, resonating hum of its engines. I confirmed that theory with marinetraffic.com, and it was a relief to know I wasn't hallucinating.

I have since moved, and started hearing "the hum" in March. I'm now about five miles inland from that port, but I hear the hum louder than I ever did from passing boats. I can't match it to any specific marine traffic, although the sound is nearly identical. It could be nautical, or it could be some other industrial noise, or it could be something like tinnitus.

My wife can't either either sound.